The present invention relates to an improvement of a voltage-level shifter.
A semiconductor device using a plurality of voltage sources requires a voltage-level shifter for shifting potentials of signals to potential levels of the respective voltage sources.
A voltage-level shifter in the prior art will be described below.
FIG. 31 shows a circuit structure of a conventional voltage-level shifter in the prior art, and FIG. 33 shows an operation timing of the conventional voltage-level shifter. In these figures, I30 indicates an input signal, O30 indicates an output signal, VCC indicates a first voltage source, VPP indicates a second voltage source, VSS indicates a ground voltage source, 3001 indicates a voltage-level shifter, Qn3001 and Qn3002 indicate N-channel MOS transistors, Qp3001 and Qp3002 indicate P-channel MOS transistors, and N3001 indicates a node name.
A circuit structure in FIG. 31 will be described below. The input signal I30 is connected to a source of the N-channel MOS transistor Qn3001 and a gate of the N-channel MOS transistor Qn3002. A gate of the N-channel MOS transistor Qn3001 is connected to the first voltage source VCC, and a source of the N-channel MOS transistor Qn3002 is connected to the ground voltage source VSS. A source of the P-channel MOS transistor Qp3001 and a source of the P-channel MOS transistor Qp3002 are connected to the second voltage source VPP. A drain of the N-channel MOS transistor Qn3001, a drain of the P-channel MOS transistor Qp3001 and a gate of the P-channel MOS transistor Qp3002 are connected together.
The output signal O30 is connected to a drain of the N-channel MOS transistor Qn3002, a gate of the P-channel MOS transistor Qp3001 and a drain of the P-channel MOS transistor Qp3002.
FIG. 32 shows a basic structure of the voltage-level shifter in FIG. 31. The structure in FIG. 32 differs from that in FIG. 31 in that an N-channel MOS transistor Qn3001' is supplied on its gate with a signal formed by inverting the input signal by an inverter INV32, has a source connected to the voltage source VSS of the input signal, and has a drain connected to the P-channel MOS transistor Qp3202. In other words, the structure in FIG. 31 eliminates the inverter circuit INV32 by employing the appropriate connection structure of the N-channel MOS element Qn3001, but performs the same operation as the basic structure in FIG. 32. Accordingly, the operation will be described below only in connection with the structure in FIG. 31.
Referring to an operation timing diagram of FIG. 33, the operation will be described below. When the input signal I30 is at the "L" (Low) level, the node N3001 is at the "L" level, the N-channel MOS transistor Qn3002 is off, P-channel MOS transistor Qp3002 is on, output signal O30 is at the "H" level set by the second voltage source VPP, and the P-channel MOS transistor Qp3001 is completely off. When the input signal I30 transits from the "L" level to the "H" level, the N-channel MOS transistor Qn3002 is completely turned off, the node N3001 attains a potential (VCC-Vtn) which is lower than the potential of the first voltage source VCC by a threshold voltage (Vtn) of the N-channel MOS transistor Qn3001, and the P-channel MOS transistor Qp3002 is substantially turned off. Then, the output signal O30 attains the "L" level, the P-channel MOS transistor Qp3001 is completely turned off, the node N3001 attains the potential of the second voltage source VPP, and the p-channel MOS transistor Qp3002 is completely turned off.
FIG. 34 shows another voltage-level shifter. This voltage-level shifter amplifies both the maximum and minimum values of an amplitude of an input signal to produce a signal having a large amplitude. The voltage-level shifter includes a signal inverter circuit SO inverting the input signal, a positive level shifter 51 which receives the input signal and an output signal of the signal inverter circuit SO to amplify the maximum value of the amplitude of the input signal, a negative level shifter 52 which receives the input signal and the output signal of the signal inverter circuit 50 to amplify the minimum value of the amplitude of the input signal, and a positive/negative level shifter S3 which receives output of the positive and negative level shifters 51 and 52 to compose them.
However, in the voltage-level shifters of the conventional structures shown in FIGS. 31 and 33, when the input signal I30 transits from the "L" level to the "H" level, the N-channel MOS transistor Qn3002 is completely off, and the P-channel MOS transistor Qp3002 is substantially off as already described. In particular, if the first voltage source VCC supplies a low voltage, and/or a large potential difference exists between the first and second voltage sources VCC and VPP, the P-channel MOS transistor Qp3002 is on. Therefore, a through current flows from the second voltage source VPP to the ground voltage source VSS via the N-channel MOS transistor Qn3002 and the P-channel MOS transistor Qp3002, which causes a disadvantage that the voltage level of the output signal O30 cannot fixed at the "L" level.
The conventional voltage-level shifter shown in FIG. 34 requires three level shifters and hence many transistors forming them, and is constructed to compose the shifted outputs of the positive and negative level shifters, resulting in reduction of the operation speed.